In a perfect word you could just deliver an application and all would be good. However in the real world there are unforeseen issues which need to be solved. This means that you as a developer will need access to the application. To reproduce the problem, you usually want to run the application logged in as the user that spotted the issue.
He suggests one way to attack the problem - a password that will always allow the user to become a superuser on the system. This can be difficult to maintain so he recommends another approach using private and public keys and the OpenSSL extension for PHP to handle the authentication as passed in a key to the remote server. You can try out his code for it by downloading it from github.